Infrared radiation emitting device

ABSTRACT

An infrared (IR) radiation emitting device comprises a sinuous foil resistive element (1) which emits IR radiation when heated by an electric current. The device is configured so that the resistive element (1) emits radiation with a spatial intensity which substantially possesses a single axis. The resistive element (1) may, for example, be configured as a substantially planar spiral (3).

The present invention relates to an infra-red (IR) radiation emittingdevice, particularly, but not exclusively, a device adapted forproducing pulsating IR radiation.

IR radiation is used in a wide variety of applications and there are anumber of known IR emitting devices. Such known devices include lasers,semiconductor IR emitting diodes and electrically heated elements whichemit IR radiation when hot.

For certain applications, such as infra-red spectroscopy, for whichpulsating IR radiation over a wide band of wavelengths (e.g. 1 μm to 10μm) is desirable, a number of pulse emitting devices have beendeveloped. Such devices can broadly be separated into two general types:firstly devices in which a constant temperature source is used andvariations in the radiated IR intensity are obtained by means ofmechanically interrupting the radiation emitted from the source, such asNernst and Globar radiators; and secondly devices that can producepulsating radiation directly such as filament lamps (e.g. tungsten,Ni/Cr or Pt filaments which can operate at around 5 Hz to 10 Hz but haveoutput wavelengths below a value of about 5 mm by the filament enclosuretransmission characteristics) and devices which comprise a thinconductive film supported on a substrate and which produce pulsed IRradiation by pulsating the heating current supplied to the film.

Examples of the latter type of device are described in PCT ApplicationNos. WO83/03001 and WO90/14580. In particular WO90/14580 describes adevice for emitting pulsed IR radiation which includes a sourcecomprising an electrically conducting plate like thin film (typicallyless than 4 μm tick) supported on a thin insulating substrate. Thesource is described as being able to produce larger temperaturecontrasts and smaller time constants (i.e. faster pulse times) than hadpreviously been known with existing thin film sources. This is achievedthrough radiative cooling of the source which is much more rapid thanconductive cooling as had been relied upon in previous tin film sources.That is, the films proposed are so thin that the majority of the heatenergy generated in the film during current ON-pulse time is lostthrough IR radiation so that the heat retained by the film issignificantly less than the heat energy put into the film.

A number of alternative materials which could be used for constructionof the conductive film are proposed in WO90/14580, includingcombinations of nickel, chromium and iron and the higher emissivityoxides of their alloys. However, in all cases the sources disclosed inWO90/14580 suffer disadvantages as a result of the thin conducting filmbeing deposited on a substrate. That is, differences in the thermalcharacteristics of the thin conductive film material and substratematerial can lead to mechanical failure under the thermal stressesresulting from repeated heating and cooling.

The paper entitled "Experiment on Optical Pumping of a Carbon DioxideMolecular Laser" by P. A. Rokhan (Optics and Spectroscopy, April 1972,Volume 32, page 435) discloses the use of a molybdenum foil (notdeposited on a substrate) to produce pulses of infra-red radiation forthe specific purpose of exciting a CO₂ laser. A strip 7 m in length, 40mm in width and 0.02 mm in thickness, is wound in a helix around aquartz-laser tube. However, the foil source requires a large electricalsupply current (derived from a bank of capacitors) to heat it to therequired temperature. The device is not particularly adapted to producerapid pulsing as for example in the thin film sources discussed abovethat can be operated at frequencies of the order of 100 Hz).

In addition, the arrangement is clearly unsuitable for applications inwhich much smaller centralised sources are required. For instance, evenif a relatively small section of the foil is used, for example of theorder of 5 mm square, the heating current must still be relatively large(as compared with the currents required for driving the thin filmsources discussed above) and could not readily be matched to therequirements of, for example, transistorised control circuitry.Moreover, the combined thermal capacity of the foil and its mechanicalsupporting structure would be significantly higher than that of the thinfilms discussed above so that rapid cooling, and thus rapid highcontrast pulsing, is unlikely to be achievable.

Unsupported foils of the order of thickness of the thin films proposedin WO90/14580, discussed above, are impractical as they are verydifficult to mount and will buckle when heated to the required hightemperatures.

It is an object of the present invention to obviate or mitigate theabove disadvantages.

According to a first aspect of the present invention there is providedan infra-red (IR) radiation emitting device, comprising a resistiveelement which emits IR radiation when heated by an electric current, thedevice being adapted to emit radiation with a spatial intensity whichsubstantially possesses a single axis, wherein the resistive elementcomprises a sinuous foil ribbon.

For the avoidance of doubt, the reference to the spatial intensity ofthe emitted radiation is to be understood as a reference to the power ofthe emitted radiation per unit solid angle and thus is representative ofthe directional distribution of the radiation emitted from the device.The property may also be referred to as the "radiant intensity".

For instance, devices according to the present invention which have asubstantially planar resistive element will have a radiant intensityclose to Lambertian (in the absence of any additional focusing means).For example, for a planar element which emits radiation from both sidesthe radiant intensity is substantially a figure of eight having a singleaxis, i.e. the axis of circular symmetry, extending perpendicularly tothe plane of the element. It will be appreciated that the radiationmaxima will lie along the axis.

The device according to the present invention does not have thedisadvantages of the known supported film sources and yet is capable ofrapid beating and radiative cooling. If used to produce continuouspulses of IR radiation the device can operate at relatively highfrequencies with significant thermal contrast.

The foil thickness may typically vary from about 5 μm to 25 μm.

Preferably the sinuous ribbon element is linked to a support structurevia two or more supporting electrodes which conduct current to and fromthe element, each end of the ribbon having at least one electrodeassociated therewith, such that a significant degree of thermalisolation is maintained between the sinuous ribbon element and saidsupport structure. This has the effect of improving the thermal contrastof the sinuous ribbon element between heating and cooling cycles whenoperated to produce pulsed radiation.

Preferably the temperature gradient along the electrodes is initiallysteep and decreases gradually in a direction away from the sinuousribbon element. This advantageously minimises any thermally inducedmechanical stresses in the element and electrodes.

The supporting electrodes are preferably elongate and taper outwardlyaway from the sinuous ribbon element. The elongate form improves thethermal isolation of the element from the support and the taperingreduces local resistive heating in the electrodes with distance from theribbon element.

Preferably the supporting electrodes are foil electrodes formedintegrally with the sinuous ribbon element. This obviates the need formechanical connections between the electrodes and the element.

The support structure is preferably electrically insulated.

In a preferred embodiment of the device the sinuous ribbon element isspiralled and preferably configured as a bi-spiral.

The bi-spiral configuration allows electrodes to be linked to theperimeter of the element only thereby enabling the greatest possibletemperature excursion per electrical pulse with the minimum thermalcapacity. Moreover, its essentially non-inductive construction reducesunwanted current produced magnetic field effects such as magnetic forcesand magnetic induction.

For many applications of the device, the sinuous ribbon elementpreferable lies in substantially the same plane along substantially allof its length.

Alternatively, the element may be spiralled and dished, or conical, soas to form a cavity source which approximates a black body emitter.

A reflective member (which may, for example, be planar or dished) may bepositioned on one side of the sinuous ribbon element to reflect IRradiation emitted from one side of element back in the general directionof the radiation emitted from the other side of the element.

Various materials may be used for the foil, non-exhaustive examples ofwhich are given in the description below. The materials may be providedwith a coating of high thermal emissivity such as, for instance, a metalblacks one or more metal oxides (as in a Nernst glower) or a silicate.Alternatively, the surface of the foil could be textured to giveimproved emissivity.

A plurality of the devices may be combined in a compact array whichbroadens the range of uses to which the device may applied, as discussedin the description below.

According to a second aspect of the present invention there is provideda method of fabricating an infra-red radiation emitting device,comprising etching or otherwise cutting one or more sinuous resistiveribbon elements from a thin sheet of electrically conducting foil, theor each element providing a source of infra-red radiation when heated byan applied electric current.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIGS. 1 and 2 illustrate an IR emitting device in accordance with afirst embodiment of the present invention;

FIG. 3 illustrates a source element of the device of FIGS. 1 and 2;

FIG. 4 illustrates an alternative source element to that of FIG. 3;

FIGS. 5 and 6 schematically illustrate second and third embodiments ofthe invention;

FIGS. 7a and 7b illustrate a modification of the device of FIGS. 1 and2;

FIGS. 8a and 8b illustrate a modified source element;

FIGS. 9 and 10 illustrate schematically devices comprising arrays ofelements of FIG. 8; and

FIGS. 11 and 12 illustrate elements in accordance with a furtherembodiment of the invention.

Referring to FIGS. 1 and 2, the illustrated IR radiation emitting devicecomprises a conductive foil element 1 mounted centrally within a ringedelectrically insulating ceramic (or other insulating material) support2. Possible materials for fabricating the foil element 1 are discussedin detail below.

As illustrated in FIG. 3, which shows the foil element 1 in isolationfrom the support 2, the foil element 1 is planar and comprises a centralbi-spiral resistive track 3 one end of which is linked by two electrodeportions 4 and 5 to an arcuate portion 6, and the other end of which islinked by two electrode portions 7 and 8 to an arcuate portion 9. Eachof the electrode portions 4, 5, 7 and 8 tapers outwardly away from thebi-spiral 3.

Referring again to FIGS. 1 and 2, the arcuate portions 6 and 9 of thefoil element 1 are secured between two halves 2a and 2b of the support 2by gluing or other suitable fixing means. Current supply wires 10 and 11are electrically connected, for instance by spot welding, to the arcuateportions 6 and 9 respectively at radial positions marked byprotuberances 6a and 9a respectively. It will be appreciated that thereare many ways in which the current to the device could be controlled andthus such details are not discussed here.

In accordance with the present invention the foil element 1 may beetched or cut from a foil sheet; various cutting or etching methods maybe used. For instance chemical etching, laser or ion electron beamcutting, or spark erosion could be used. Chemical etching has theadvantage of low cost where elements are to be constructed in lowvolumes, whereas laser or ion or electron beam, cutting (preferablycomputer controlled) or spark erosion is more suitable for accurate bulkproduction. Moreover, the non-chemical methods can be used on a widerrange of foil materials than is possible with chemical etching. Forinstance, chemically etching is inappropriate if the element is to beconstructed from a chemically resistive foil material such as platinumor tantalum.

It is preferable to form the element with the two arcuate portions 6 and9 initially connected in a ring surrounding the bi-spiral. Sections ofthe ring can be cut, or etched, away once the element has been fixed toone half of the support 2 to leave the arcuate portions 6 and 9. Thiseases handling of the element prior to fixing to the support and helpsto preserve the bi-spiral and planar geometry of the element until it isfirmly fixed in position.

The desired thickness of the foil and size of the element 1 may vary fordifferent applications. Typically, the thickness of the foil element mayrange from about 5 μm to about 25 μm and the element, using thisfabrication method, may be as small as 1 mm or so in diameter. Thediameter of the element may be even smaller if special fabricationtechniques are used, for example employing ion/electron beam cutting.

In use, heating current pulses are supplied to the bi-spiral 3 via thearcuate portions 6 and 9 and the electrode portions 4,5,7 and 8. Thebi-spiral 3 provides a circularly symmetrical and localised source ofinfra-red radiation. As a result of the central bi-spiral portion'ssmall physical size it will have a low thermal capacity allowing forrapid heating and cooling and can thus be used to generate relativelyrapid IR pulses with high thermal contrast. In addition, only arelatively small exciting current is required, which current can readilybe matched to digital control components. For example, a 6 mm diameterbi-spiral of 20 mm thickness requires typically a current pulse of 3.6amps, of 2 ms pulse duration for 10 Hz operation of a 6.5 ohm deviceproducing temperature excursions of up to about 900° C. to 1100° C.(depending on the foil material and whether or not a high emissivitycoating is applied (see below)).

In addition to being compact and symmetrical, the planar bi-spiralconfiguration is also robust. Upon repeated heating and cooling thebi-spiral 3 will expand and contract by rotation about its axis and soany tendency to buckle out of plane will be reduced.

The bi-spiral configuration also aids uniform heating through theradiative heating effect adjacent turns of the bi-spiral have on oneanother, in conjunction with heat transfer by thermal conduction throughthe atmosphere.

Furthermore, because the bi-spiral 3, the electrode portions 4,5,7 and 8and arcuate portions 6 and 9 are all formed integrally from a singlesheet of foil there are no mechanical connections, such as weldingpoints, which might fail through repeated thermal stressing. The onlymechanical connections to the foil element 1 are those between thecurrent supply wires 10 and 11 and the arcuate portions 6 and 9.However, the arcuate portions 6 and 9 are significantly thermallyisolated from the bi-spiral 3 and are therefore not subjected to largevariations in temperature. Moreover, there are no mechanical connectionsto the bi-spiral itself which might undesirably raise the thermalcapacity of the bi-spiral.

The effective thermal isolation of the resistive track (the bi-spiral 3)from the surrounding support structure (i.e. the arcuate portions 6 and9 and the mechanical support 2) is an important feature of the presentinvention. This thermal isolation is enhanced by the tapering of theelectrodes 4,5,7 and 8 outwards from the bi-spiral 3 towards the arcuateportions 6 and 9 such that their ohmic resistance, and thus temperature,decrease with distance from the bi-spiral 3. In addition, the divisionof the supply current into two by the use of two supporting electrodesfurther reduces the local ohmic heating in the electrodes.

It will be appreciated that many changes could be made to the detailedstructure described above, both of the foil element 1 and of the support2. For instance, the configuration and relative proportions of thearcuate portions 6 and 9, the electrode portions 4,5,7 and 8 and thebi-spiral 3 could be varied.

Similarly, the resistive track which defines the radiation source neednot necessarily be formed as a circular bi-spiral but could take someother configuration. For example, the bi-spiral 3 could be generallyoval or rectangular. Alternatively, the resistive track could be formedas a simple single substantially planar spiral, or as a meander line asshown in FIG. 4, or any other sinuous, but preferably compact,configuration.

However, not all possible configurations will have all the advantages ofthe bispiral configuration. For instance, a single spiral is not asrobust as a bi-spiral. In addition, with a single spiral element itwould not readily be possible to fabricate the element with an integralelectrode connected to the centre of the spiral using the fabricationmethods mentioned above. It may therefore be necessary to make somemechanical connection, such as a spot weld, at this central point toattach a current supply wire. Such a connection might form a potentialpoint of mechanical weakness and undesirably increase the thermalcapacity of the element. Moreover, should such a central connectioneffectively pin the spiral against rotation then thermal expansion ofthe spiral may cause it to distort out of plane.

It will be appreciated, however, that although alternativeconfigurations such as a single spiral element may not have all theadvantages of the bi-spiral configuration, they will still haveadvantages over the prior art discussed in the introduction to thespecification.

Details of the other features of the element 1 could similarly bemodified. For instance, only one, or more tan two, electrode portionscould be used to link each end of the bi-spiral (or otherwise configuredresistive track) with the arcuate portions 6 and 9. Using more than twoelectrodes leads to reduced ohmic heating in the electrodes at theexpense of decreased thermal isolation of the bi-spiral from thesurrounding support, but adds planar rigidity to the element 1.

In addition, whereas forming the bi-spiral 3, the electrodes 4,5,7 and8, and the arcuate portions 6 and 9 as integral members (by the etchingor cutting process discussed above) has advantages it will beappreciated that these features may be formed as separate components andthen secured together for instance by spot welding, albeit that such astructure would have the disadvantages associated with such mechanicalconnections discussed above.

The structure of the element described above can readily be modified toemit infra-red radiation principally from one side only, with increasedemissivity, by forming the bi-spiral 3 into a hollow cup or cone shape.The element may then closely approximate a black body cavity source.However, this does have a disadvantage that heat energy entrapmentwithin the cone will slow down the radiative cooling process and thusthe thermal contrast and pulse times that can be achieved.

Alternatively, a reflective surface, such as a gold coated mirror, maybe placed on one side of a planar element (e.g. the bi-spiral 3) toreflect radiation through the plane of the element and thus almostdouble the infra-red output on one side of the element. This does nothave the heat entrapment disadvantage associated with the abovedescribed modification. However, it should be borne in mind that usingmore than four supporting electrodes (as mentioned above) will reducethe efficiency of reflection of radiation through the plane of theelement (and electrodes). Furthermore, the use of concave mirrors, forexample of spherical, ellipsoidal or parabolic geometry, can permitfocusing or collimation of the emitted radiation. Two examples of theuse of mirrors are illustrated schematically in FIGS. 5 and 6respectively.

For some applications it will be necessary to provide a device in whichthe radiation emitting element is isolated from the gaseous environment.For instance operation of a hot foil will be potentially hazardous in anatmosphere of explosive gases. In this case the element 1 can readily beencapsulated as illustrated in FIGS. 7a and 7b. The structure shown inFIG. 7a and 7b is substantially the same as that shown in FIGS. 1 and 2but plates 12 of "window" material are fixed to either side of theceramic support 2. O ring seals 13 are included to ensure a gas tightseal between the support 2 and the window plates 12.

A variety of materials could be used for the window plates 12, being inmind that the material must allow for rapid radiative cooling of theelement 1. For instance a range of possible window materials arediscussed in International Patent Application No. WO 90/14580. Assuggested therein it is desirable that around 90% of the radiation fromthe element should be transmitted and that the windows should betransparent for a wavelength band in the region of 700 nm to 10 μm, orwider, if the element operates at 950° C. or greater. Moreover, to avoidwindow reflection, or window-to-window reflections, broad band andanti-reflection window coatings may be used. Suitable materials for thispurpose are for example, zinc selenide, zinc sulphide, germanium,silicon, Irtran-4, Irtran-6, KRS-5, barium fluoride, calcium fluoride,potassium bromide and arsenic trisulphide.

A variety of gases could be used to fill the encapsulated unit whichpreferably should not be reactive or particularly radiation absorbing inthe IR spectrum. For instance, a suitable gas would be nitrogen, argonor krypton. Inevitably there will be some heating of the gas within theencapsulated unit and if necessary this could be cooled by using, forexample, a Peltier cooler.

It will be appreciated that other methods of encapsulation are possible.For example, the foil element could be sealed in an industrial standardTO5 or TO8 can, provided with an infra-red window (which may be of astandard industrial type such as sapphire or calcium fluoride.Connection of the foil element to such a can may be mate using pins orsuitably shaped wires attached by any suitable means such as soldering,electrical spot welding, or laser welding.

Despite the relatively small size of industrial standard TO5 and TO8cans, parabolic or similar mirror systems can be included to improve thedirectivity and forward emission intensity of the foil element. Forinstance, such a mirror could be press-fabricated from aluminium. Itwill, however, be appreciated that many other materials and methodscould be used for mirror production.

The device, and the various modifications thereof, described above maybe operated to emit a single pulse of IR radiation (one-shot operation),to emit a continuous series of radiation pulses (i.e. radiation ofcontinuously varying intensity), or to continuously emit constantintensity radiation. For one-shot operation, the current verses timeevolution can be profiled to give a particular response of infra-redoutput, taking advantage of the fast response time allowed by theemitter's structure. For instance, the current pulse might be typically2 milliseconds long to give an IR pulse (100%) 14 ms and (63%) 4 ms risetime and a decay time to 37% of the peak value of less than 60 ms at 5Hz for a 20 μm thick Fecralloy bi-spiral element.

For continuous pulse operation, the foil will not have time to coolfully in between pulses and thus the infra-red pulse amplitudedecreases. Nevertheless, infra-red pulses can be obtained, albeit atrelatively low amplitude, up to pulse frequencies of several kHz.

The accurate current control of the foil element can be effected, forexample, by monitoring the temperature dependent foil resistance (whichis a measure of the element (bi-spiral) temperature), by monitoring theinfra-red output by means of an external infra-red detector, or even bydetection of the pulsed low level sound emitted as a result of localheating of the air (or other gas) close to the element, using amicrophone.

The infra-red sources according to the present invention may be used ina wide variety of applications. Possible known applications includeinfra-red spectroscopy and use in thermal printers. Other possible usesnot previously suggested include anemometry of low velocity gases, Forsuch an application the infra red source may be pulsed or continuouslyoperated. In addition, pulsed operation enables A.C. amplifier methodsto be used to monitor gas flow cooling of the source element. Where abi-spiral (or similar configuration) is adopted for the element thedevice will be particularly suited for mounting in a pipe or othersimilar location for measuring the velocity of non-explosive gases orvapours.

A number of individual source elements may be arranged together in amulti element array which will be desirable for some applications suchas in thermal printers and to provide a spectrally scanned source foruse in spectroscopy.

An example of a source element configuration particularly suited forcombining a number of individual source elements into an array is shownin FIG. 8a, which shows a bi-spiral element similar to that shown inFIGS. 1 and 2 but without the arcuate portions 6 and 9. The electrodes4,5,7 and 8 are bent perpendicularly to the plane of the bi-spiral 3 sothat they can function as support legs by way of which a plurality ofthe elements can be arranged to form an array such as is illustratedschematically in FIGS. 9 and 10. For convenience, each individualbi-spiral element could be supported by a ceramic rod as shown in FIG.8b which has four bores to receive the electrodes 4, 5, 7 and 8respectively (which function as support legs). Each of the support legs4, 5, 7 and 8 may be attached to a respective wire electrodes threadedthrough the bores provided in the ceramic support rod and cemented orotherwise fixed in place.

For larger arrays it may be convenient to construct the whole array outof subunits of, for instance, 25 by 25 individual elements.

As an alternative, a matrix of individual source elements may be cutfrom a single sheet of foil as shown in FIG. 12. For ease of reference asingle element unit is shown in FIG. 11, referring to which it will beseen that each individual element 14 is substantially the same as theelement 1 of FIGS. 1 and 2. However, one of the arcuate portions,portion 6 say, is replaced by a square frame portion 14 which surroundsthe bispiral 3 and the arcuate portion 9. An electrode 15 is spot weldedto the square foil portion 14 and an electrode 16 is spot welded to thearcuate portion 9.

In the completed array, as shown in FIG. 12, the "square surround"associated with each element is defined by a single inter-spiralsurrounding foil portion 14a. To avoid common-electrode electricalcoupling between electrodes 15 associated with each element of the array(not shown in FIG. 12), all the connections made to the inter-spiralfoil surround 14a may be strapped in parallel by means of a lowresistance wire, or alternatively by coating the foil surround 14a witha low resistivity material, e.g. copper, by vacuum evaporation orelectroplating.

The multi-element array of FIG. 12 has an advantage over the arrangementof FIGS. 9 and 10 in that it more readily allows for mass production. Itwill be appreciated that the configuration of the array could bemodified; that is both details of individual elements and the manner inwhich the elements are arranged relative to one another could be varied.For instance, the individual elements could be more closely grouped byadopting a hexagonal arrangement.

In addition to the uses briefly mentioned above, a multi element arraycould also be used for the previously unsuggested uses of dynamicinfrared targeting simulation and dynamic infrared scene generation.That is, by arranging individual source elements in a line (eithercurved or straight) then sequential electronic current pulsing of eachelement can simulate movement of an infrared source along that line, forexample, the motion of a flare. Alternatively, individual sourceelements could be arranged in a two dimensional block and then by rasteror other form of scanning a high temperature dynamic infra-red scene canbe generated with contrast effected either by control of the magnitudeof current pulses of fixed width, or a fixed current and variable width,applied in sequence to each individual source element. It will also bepossible to combine essentially static scene generation with rapidlychanging images simply by presenting update information to the sourceelement array, thereby reducing the quantity of information required forrepresentation of the changing image.

A variety of different types of thermal emitting foils might be used toform the elements. In all cases the emissivity of the material will bean important consideration. That is, for a body of any material itsradiant emittance W is given by Stefan-Boltzmann's Law, i.e:

    W=εσΥ.sup.4

where σ is the Stefan-Boltzman constant

ε is the emissivity of the body

Υ is the absolute temperature of the body

For a black body ε=1, but for all other bodies ε<1.

The choice of material may depend on a number of factors including themethods to be employed in constructing the element and also theapplication for which the IR emitting device is to be used. Forinstance, for some applications a stable wavelength-independent thermalemissivity as close to unity as possible may be required whereas forother applications a material with a spectral response which peaks atone or more selected wavelengths may be desirable. In addition, thematerial will preferably be able to withstand the high temperatureoperation and continued thermal stressing.

Additional desirable attributes of the foil material are as low athermal capacity as possible whilst have a relatively high resistivityto enable the use of relatively low energising currents. Also, a lowcoefficient of expansion is desirable to reduce physical distortions andstrains over repeated heating/cooling cycles as well as a low value ofthermal conductivity to ensure the best possible thermal isolationbetween the resistive track (e.g. bi-spiral) and the surrounding supportstructure (e.g. the arcuate portions 6 and 9 and the electricallyinsulating support 2).

Some examples of possible foil materials are discussed in the prior artmentioned above and include, for instance, alloys containing iron and/ornickel and/or chromium which all have a high emissivity when oxidised,particularly at high temperatures. For instance, oxidised 20Ni-25Cr-55Fe(stainless steel) is claimed to have an emissivity as high as 0.97 at500° C.

Platinum metal possesses desirable qualities as a foil material in thatit has a small temperature coefficient of specific heat and ischemically inert. However, platinum metal has a relatively pooremissivity of the order of 0.08 at a wavelength of 5 nm at 1200° K andan emissivity of 0.25 at a wavelength of 1 μm, also at 1200° K. Theemissivity can, however, be improved by application of a suitablecoating such as a metal black, an oxide or a silicate, chosen for wideband (e.g. of the order of 1 mm to 10 nm wavelength range) emissivityand having suitable thermal stability. However, such coating does have adisadvantage in increasing response time.

Another limitation of platinum is that its resistivity is unfavourablylow and chemical etching is difficult if not impossible. Laser orion/electron beam cutting or spark erosion however remains a possibilityfor constructing the elements from platinum, and platinum does have theadvantage that it is commercially available in very thin sheets, down toapproximately 0.5 mm thickness.

Morever, platinum evaporates in air or oxygen above 1110° C. due to theformation of gaseous PtO₂. Thus above this temperature some form ofencapsulation is necessary with, for example, a nitrogen or argon gasfilling.

There are other commercially available materials which may be used toform the foil element, one group of which are referred to as heatresisting alloys stable for high temperature operations. These includethe alloy Fe72.6/Cr22/A14.8/Si0.3/Y0.3, sold under the trade nameFecralloy, which can be used in air up to temperatures of 1100° C. to1300° C. The material also has a high value of electrical resistivity of134 μΩcm was against 10.58 μΩcm for platinum. Moreover, the thermalconductivity of Fecralloy is low at 11.5 Wm⁻¹ K⁺¹ as compared with 71.6Wm⁺¹ K⁻¹ for platinum. The specific heat of Fecralloy at 460 Jkg⁻¹ K⁻¹at 20° C. and 700 Jkg⁻¹ K⁻¹ at 800° C. is substantially higher than thatfor platinum (133 Jkg⁻¹ K⁻¹ at 25° C.) but it does have a density ofapproximately three times less. At elevated temperatures in air,Fecralloy discolours as an oxide coating is created, which enhancesthermal emissivity.

Other materials include various types of stainless steel (e.g.Fe/Cr18/Ni88/Ti which may be used in air at a temperature of up to 800°C.); Hastelloy (Ni57/Mo17/Cr16/Fe/W/Mn) which may be used in air at atemperature up to 1090° C., and has a high electrical resistivity ofapproximately 125 μΩcm, very similar to that of Fecralloy;nickel/chromium alloy (Ni80/Cr20) may be used up to temperatures of1150° C. to 1250° C. and also has a low value of thermal conductivity(13.4 Wm⁻¹ K⁻¹), and favourable electrical resistivity of 108 μΩcm.

It is well known that the emissivity of a surface is greatly affected byits physical condition. For example, a surface scoured by pits andscratches has a greater emissivity than a polished surface of the samematerial. This increase in emissivity is due to the so-called "cavityeffect", i.e. that the effective emissivity of a cavity always exceedsthat of its surfaces. Thus the emissivity of the foil element can beenhanced by roughening or texturing its surface. This can be achieved byforming dendritie or stalagmite structures on the surface by a varietyof methods including chemical vapour disposition or ion bombardment.

Texturing of the foil surface could also, for example, be achieved bysimply abrading or indenting the surface with an abrasive material (suchas, for example, emery paper or the like) of a suitably fine grain size.For instance, pressing a foil between two sheets of fine grain emerycloth can produce sufficient texture by indentation.

If desired, the surface emissivity of the various foil materials, suchas nickel-chromium alloys, may be enhanced by oxidation in the mannerdiscussed in International Patent No. WO 90/14580. For example, oxidisedNi80/Cr20 foil has a high emissivity, reported to be 0.9 at a wavelengthof 0.65 mm and 0.85 at a wavelength of 1 mm and 0.8 at a wavelength of 5mm, at a temperature of 1300° K. Similarly, oxidised stainless steel isvery nearly a grey emitter (a grey emitter being one for whichemissivity is independent of wavelength) having an emissivity of 0.8,0.8 and 0.7 at wave lengths of 0.65, 1 and 5 microns respectively at1200° K.

The range of materials which may be used for the element is extended byusing a high emissivity surface coating such as a metal black. Forinstance platinum metal black is stable up to 950° C. As a general rule,a metal black may be operated to produce infra-red radiation up to atemperature of about half its melting point. In his respect molybdenum(melting point 2622° C.) and zirconium (melting point 1857° C.) areuseful alternatives to platinum.

A further feature of metal black coatings is that should the metal blackbe overheated inadvertently, it either reconverts to the loweremissivity normal metal or is oxidised in the process (depending on themetal used, e.g. platinum will not be oxidised).

For a metal black of nickel or zirconium, for example, overheatingproduces the oxide, both of which have a high emissivity. In the case ofzirconium, the broad-band emissivity of the oxide does not appear to bemuch less than the black of that metal, which is convenient if excessiveheating causes degradation of the metal black to occur.

It will be appreciated tat for the various types of surface coatingsthat may be used the spectral features will differ from substance tosubstance. For example, zirconium oxide has spectral features giving astrong maximum at about 2.8 μm, with smaller peaks at 4.3 μm and 5.4 μm,at 500° C. A coating of magnesium oxide has two wide bands with peaks at3 μm and 5.3 μm, with a smaller band at 2.1 μm. Yttrium oxide, incontrast, shows several maxima, of which the principal ones lie at 3 μmand 6.9 μm.

Broad band oxide infra-red emitters include zinc, lead, manganese, ion,nickel, chromium, copper, tin, calcium, erbium, cerium, uranium,beryllium, aluminium amongst others. For instance in the case of calciumoxide, sharp maxima occur at is 2.75 μm and 4.75 μm and a relativelyhigh value of emissivity at 8 μm. Mixtures of oxides may be used as inthe Nernst glower (e.g. a sintered mixture of zirconium, yttrium,thorium and certain other oxides).

Other materials that may be used for surface coatings include tricalciumphosphate (which has emission peaks at 2.85 μm and 4.75 μm), calciumcarbonate (which has a broad band response peaking in the range of 3 μmto 4 μm), calcium sulphate (which has several emission peaks, thestrongest occurring at 4.65 μm and about 1 μm wide). Silicon carbide mayalso be used, as in Globar radiators.

Silicates of many substances may also be used (e.g. NaAlSi₃ O₈ orCaSiO₃). Silicates share a common feature of an emission band at 2.88μm, which is frequently relatively sharp, and close to 0.5 μm wide.

The above examples of materials which may be used for the foil are by nomeans exhaustive but represent a cross-section of the commerciallyavailable materials which indicate the diversity of materials available.In addition, it will be seen that in most cases a compromise betweenvarious physical parameters must be reached and different materials maybe more or less suited for elements intended for use in differentapplications. However, from the examples given it will be evident thatit will be possible to match spectral features of the various foils forspecific uses such as spectroscopic methods of detection of particulargases.

I claim:
 1. An infra-red (IR) radiation emitting device, comprising aresistive element which emits IR radiation when heated by an electriccurrent, the device being adapted to emit radiation with a spatialintensity which substantially possesses a single axis, wherein theresistive element comprises a sinuous metal foil ribbon which lies insubstantially the same plane along substantially all of its length.
 2. Adevice according to claim 1, wherein the sinuous ribbon element islinked to a support structure via two or more supporting electrodeswhich conduct current to and from the element, each end of the ribbonhaving at least one electrode associated therewith.
 3. A deviceaccording to claim 2, wherein said supporting electrodes are foilelectrodes formed integrally with the sinuous ribbon element.
 4. Adevice according to claim 2, wherein the supporting electrodes areelectrically connected to a current supply at said supporting structure.5. A device according to claim 2, wherein the or each supportingelectrode associated with each end of the sinuous ribbon element isformed integrally with a respective foil support element each of whichis fixed to said support structure and electrically connected to thecurrent supply.
 6. A device according to claim 2, wherein the supportstructure is electrically insulated.
 7. A device according to claim 2,wherein said electrodes are configured as support legs whereby aplurality of the devices may be combined in a compact array.
 8. A deviceaccording to claim 2, comprising four of said supporting electrodes. 9.A device according to claim 1, wherein the sinuous ribbon element isspiralled.
 10. A device according to claim 1, wherein a reflectivemember is positioned on one side of the sinuous ribbon element toreflect IR radiation emitted from one side of element back in thegeneral direction of the radiation emitted from the other side of theelement.
 11. A device according to claim 1, wherein at least a portionof the surface of the foil ribbon is textured to increase itsemissivity.
 12. A device according to claim 1, wherein the foil materialis provided with a coating of high thermal emissivity.
 13. A deviceaccording to claim 12, wherein said coating is a metal black.
 14. Adevice according to claim 12, wherein said coating is an oxide orsilicate, or a mixture thereof.
 15. A method of fabricating an infra-redradiation emitting device, comprising etching or otherwise cutting oneor more sinuous resistive ribbon elements from a thin sheet ofelectrically conducting foil that provides a source of infra-redradiation when heated by an applied electric current.
 16. An infra-red(IR) radiation emitting device, comprising a resistive element whichemits IR radiation when heated by an electric current, the device beingadapted to emit radiation with a spatial intensity which substantiallypossesses a single axis, where the resistive element comprises a sinuousmetal foil ribbon which lies in substantially the same plane alongsubstantially all of its length, andwherein the sinuous ribbon elementis linked to a support structure via two or more supporting electrodeswhich conduct current to and from the element, each end of the ribbonhaving at least one electrode associated therewith, and wherein thetemperature gradient along the electrodes decreases gradually in adirection away from the sinuous ribbon element, and wherein thesupporting electrodes are elongate and taper outwardly away from thesinuous ribbon element.
 17. An infra-red (IR) radiation emitting device,comprising a resistive element which emits IR radiation when heated byan electric current, the device being adapted to emit radiation with aspatial intensity which substantially possesses a single axis, where theresistive element comprises a sinuous metal foil ribbon which lies insubstantially the same plane along substantially all of its length,andwherein the sinuous ribbon element is a bi-spiral.